This invention relates to improvements in a charged particle beam generating apparatus of multistage acceleration type, and more particularly to an improved structure of a charged particle beam generating apparatus of multi-stage acceleration type which can operate at an acceleration voltage as high as 100 kV or more and which is suitable for use as, for example, a field emission electron gun of multi-stage acceleration type.
An example of a field emission electron gun of multi-stage acceleration type is disclosed in co-pending patent application Ser. No. 365,827 filed by the inventors on June 14, 1989.
A prior art charged particle beam generating apparatus of multi-stage acceleration type used as a field emission electron gun is described in, for example, U.S. Pat. No. 4,642,461 to Endo et al patented on Feb. 10, 1989. The disclosed electron gun includes a multi-stage acceleration tube, a plurality of acceleration electrodes arranged in cascade in the individual stages on the radially inner side of the multi-stage acceleration tube, a plurality of shield electrodes disposed in the individual stages on the radially outer side of the multi-stage acceleration tube and a plurality of dividing resistors disposed between the wall of the multi-stage acceleration tube and the shield electrodes in the individual stages for applying predetermined acceleration voltages to the respective acceleration electrodes. Both the acceleration electrodes and the shield electrodes are commonly made of a metal such as permalloy having a high permeability, so that the electric field distribution at the wall of the multi-stage acceleration tube can be made smooth and uniform thereby preventing occurrence of undesirable discharge between the acceleration stages and so that they can act as means for shielding the electron gun against an external magnetic field.
An electron gun for use in, for example, an electron microscope is required to be operable at a high voltage in view of the demand for improving the resolution. At the same time, maintainability is also required for the electron gun. Further, magnetic shielding capability is also required for the electron gun so that the path of its electron beam may not be adversely affected by an external magnetic field.
Therefore, undesirable discharge at its multistage acceleration tube must be prevented because a high voltage is applied to the acceleration electrode in each stage of the multi-stage acceleration tube. Further, the structure of the electron gun must be such as to permit easy disassembling and re-assembling of the electron gun.
With the increase in the acceleration voltage, minute discharge tends to induce occurrence of undesirably great discharge. Therefore, a more complete measure for preventing the undesirable discharge is required. Further, with the increase in the acceleration voltage, the length of the multi-stage acceleration tube must be extended in proportion to the level of the voltage. In this case, however, the magnetic shielding effect must also be improved in a relation generally proportional to the square of the length of the multi-stage acceleration tube.
In such a charged particle beam generating apparatus of multi-stage acceleration type, undesirable discharges attributable to application of a high voltage tend most frequently to occur at the joint between the cylindrical member of an electrical insulator constituting each stage of the multi-stage acceleration tube and a spacer of a metal located between the individual insulating cylindrical members so as to apply the divided acceleration voltage to each stage of the acceleration tube. The electric field strength in the area of this joint is determined by the shape of the joint and also by the shape and arrangement of the acceleration electrode and shield electrode. For the purpose of preventing occurrence of undesirable discharge, it is most important that any abrupt change should not appear in the electric field distribution so that the electric field may not be concentrated in a specific location.
In the prior art charged particle beam generating apparatus of multi-stage acceleration type in which the dividing resistors and means such as links of an electrical insulator for adjusting the position of the charged particle beam source are disposed between the wall of the multi-stage acceleration tube and the shield electrodes, the shield electrodes are spaced apart by a considerable distance from the acceleration electrodes, and the presence of the dividing resistors leads to a non-uniform potential distribution between the acceleration stages. Therefore, alleviation of undesirable concentration of the electric field in the area of each of the joints described above has had a limitation in the prior art apparatus. Also, for the same reason, the shield electrodes in the prior art apparatus have had a limited effect on shielding against the external magnetic field. Especially, with the increase in the acceleration voltage, more complete measures to deal with the high-voltage discharge and to ensure the magnetic shielding effect are required. However, the prior art apparatus have been unable to sufficiently satisfy these requirements.
In the case of disassembling or re-assembling the apparatus, the acceleration electrodes, shield electrodes and dividing resistors must be dismounted and re-mounted one by one.
In the conventional apparatus, the distance between the electrodes is generally selected to produce an averaged electric field of only about 5 kV per mm so as to prevent occurrence of undesirable discharge between the acceleration electrodes or between the shield electrodes. Therefore, the magnetic shielding effect in the prior art apparatus has not necessarily been sufficient. Further, in the prior art apparatus, the confronting surfaces of the electrodes are polished to a mirror finish so as to prevent occurrence of undesirable discharge. Therefore, the prior art apparatus has been disadvantageous in that a high cost is required for polishing the electrode surfaces, and such electrode surfaces tend to be easily damaged during the dismounting and re-mounting of these electrodes.
Although the dividing resistors need not necessarily be disposed near the multi-stage acceleration tube as employed in the prior art apparatus, they are required to be installed in the same insulating housing and to be connected to the respective acceleration electrodes in the multi-stage acceleration tube. However, connection of lead wires between the acceleration electrodes and the dividing resistors is complex and troublesome in the prior art apparatus, and contamination or fouling of the surface of the dividing resistors leads to occurrence of undesirable high-voltage discharge. Thus, the prior art apparatus has also been disadvantageous from the aspects of lead connectability and stability against the high-voltage discharge.